It is known that the addition of fertilizer blends in the application of many pesticides will improve the performance of the active ingredient. The current market standard is ammonium sulfate (AMS). It is speculated that one of the reasons for this is that the anion portion of the fertilizer blend, sulfate, will associate with the hard water cation. Therefore the anion or acidic pesticide will not associate with the hard water cation and be more available for uptake into the target species. “Data suggest hard-water cations, such as Ca+2 and Mg+2 present in the spray solution can greatly reduce the efficacy of glyphosate. These cations potentially compete with the isopropylamine in the formulation for association with the glyphosate anion.”1 Hard water with cations present in a concentration range higher than 100 ppm-150 ppm have been shown to cause a decrease in effectiveness of many pesticides.2 It is thought by some authors that the reason for the reduced activity with glyphosate is that the glyphosate anion will form insoluble salts with many hard water cations. This would be true for many anions pesticides including glyphosate, 2,4-D and glufosinate. This would also be true for acidic herbicides that could become anionic depending upon pH an example of this would be sethoxydim.3,4 
This information has lead to the common practice of glyphosate and other anionic pesticides being applied in the presence of ammonium sulfate (AMS) in the spray mixture. However, in other industries a common practice to remove hard water cations such as Ca+2, Fe+2, Mg+2 and Zn+2 is with acidic reaction with mineral acids such as nitric and sulfuric acid.5 This technology has been adapted to cation management in both soil and irrigation water and is based on the “Langelier index”.6 Cation management with phosphoric acid as a spray mixture has been tried with limited success as compared to spray mixtures containing AMS. It is speculated that the reason that phosphoric acid products do not work as well as AMS is that phosphoric acid does not completely dissociate when added to water at normal spray mixture pH ranges.8 It is therefore less reactive to the hard water cations than originally thought by the creators of these products. Other mineral acids were considered to be impractical in pesticide applications because small mistakes or misuse with these powerful acids will drop the pH of a spray solution in the spray tank below the pKa of many anionic pesticides, including glyphosate. If this occurs the pesticide will precipitate and will no longer be sprayable.
An idea was formed that mineral acid management of hard water cations would be much more efficient than AMS management of hard water cations if a mineral acid that completely dissociates in water could be used and a reliable delivery system could be devised or discovered for these types of acids. The three driving factors for this idea are: 1) Much less acid is needed to tie up the hard water cations than AMS. The amount of AMS recommended is 17.5 lb per 100 gallons of waters with 150-250 ppm hard water cations. 4 Whereas only 1.3 oz of sulfuric acid per 100 gallons is needed to neutralize the 150-250 ppm hard water cations.7 2) Sulfuric acid and nitric acid will form semi and insoluble salts with the hard water cations. Whereas AMS has only been shown to associate with these cations. It is unlikely that they form salts.3 3) AMS as a salt is hard to dissolve into the spray solutions which makes it difficult to work with. Whereas, acids are completely miscible in water.
1. The basis for the hard-water antagonism of glyphosate activity. Thelen, K. D. Weed Science v. 43 (4) 1995 pp. 541-548.
2. Weed Science Principles and Application. Anderson, WP third edition, West Publishing Co. Minneapolis, Minn.
3. Nalewaja, J. D. and R. Matysiak. 1993. Pesticide Sci. 38:77-84.
4. Role of AMS with glyphosate products. Hartzler, R. Extension Bulletin, Iowa State University.
5. David Wm. Reed. 1996. Water, media and nutrition for greenhouse crops. Ball Publishing, Batavia, Ill., ISBN: 1-883052-12-2.
6. Bohn H. L., McNeal, B. L., O'Connor, G. A., Soil Chemistry, A Wiley-Interscience Pub. John Wiley & Sons, New York, N.Y.
7. Greenhouse Product News February '99.
8. Morrison R. T. and Boyd N. B. Organic Chemistry, Allyn and Bacon, Boston, Mass.